Microporous stencil sheet and application thereof

ABSTRACT

A microporous stencil sheet for use in stencil printing and methods for making stencil master using this microporous stencil sheet are disclosed herein. The microporous stencil sheet is for use in stencil printing using a low-viscosity ink with a viscosity ranging from 0.001 to 1 Pa•s, comprises an inelastic resin film and has an air permeability ranging from 1 to 600 seconds and a thickness ranging from 1 to 100 μm. The methods for making stencil master using this microporous stencil sheet comprise the step of closing the micropores of the stencil sheet in a non-printing portion of a desired printed image.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a microporous stencil sheet foruse as a master for stencil printing, and to a method for making astencil master using the stencil sheet and a stencil master obtained bythe method.

[0003] 2. Description of the Related Art

[0004] Known stencil printing sheets (stencil sheets) used for stencilprinting include heat-sensitive stencil sheets that comprise athermoplastic film and a substrate tissue paper laminated together usingan adhesive. These stencil sheets are perforated by irradiation withinfrared rays or by the use of a thermal head (a thermal printing head)in order to make a master. As stencil printing ink, a water-in-oilemulsion ink is mainly used.

[0005] Stencil printing is effected by perforating a stencil sheet in aprinting portion of a desired printed image to make a stencil master,and passing ink from the tissue paper side through the perforatedportion of the master onto printing paper. In the following description,a printing portion refers to all parts to be printed of the printingpaper. The amount of ink transferred (hereinafter referred to as “inktransfer amount”) is controlled by, for example, the viscosity of ink,the density of tissue paper (to control the passing resistance of ink),the density of the perforated area of film, printing pressure, andprinting pressure time.

[0006] In stencil printing, large amounts of ink are transferred toprinting paper, requiring considerable time for the ink to permeate theprinting paper. Improvement in drying property of such ink is thereforerequired. Specifically, the ink is resistant against permeating thesurface of printing paper, and fingers become dirty when printed matteris touched immediately after printing. Additionally, the ink istransferred to, for example, the rubber rollers of a printing machineand then to printing paper, thereby making the printed matter dirty, ifa second or further color is printed in color printing or if the reverseis printed in duplex printing, immediately after the first printing. Dueto these problems, once the printing paper is printed in a first step,considerable time is required (e.g., from about 10 to about 20 minutes)before subjecting the work to the subsequent step. On the other hand, ifthe ink transfer amount is simply reduced, print quality is deterioratedsuch that the resulting printed image may appear blurred.

[0007] As a possible solution to the problem mentioned above, severaltechniques have been proposed in order to reduce the ink transfer amountwithout the deterioration of print quality, such as (1) a technique inwhich the density of tissue paper is increased to increase the passingresistance of ink, (2) a technique in which the element size of athermal head is decreased to minimize the area perforated in thermalhead master-making, and (3) a technique in which a product of printingpressure and printing pressure time is kept to minimum levels.

[0008] However, according to technique (1), the number of contact pointsbetween the film and tissue paper increases with the increase of tissuepaper density, and molten components of the film tend to accumulate atthese contact points, thereby deteriorating the perforating property inmaster-making, resulting in insufficient perforation. Additionally, openpore distribution of such tissue paper is difficult to control due tothe manufacturing method of tissue paper; if the paper has a largedistribution of open pores, variation in the perforation area canresult. Accordingly, the resulting perforated stencil master hasportions with high ink-permeability and with low ink-permeability,making the uniform transfer of ink difficult.

[0009] According to technique (2), by increasing the packing density ofthermal head elements, a density (resolution) of up to 600 dpi isachieved. However, the resulting perforated pores are of sizes rangingfrom about 20 to about 40 μm, and are still insufficient to control theink transfer amount, since the pore size must generally be 20 μm or lessin order to reduce the ink transfer amount in printing using alow-viscosity ink.

[0010] According to technique (3), the mean ink transfer amount can becontrolled. However, because the tissue paper has a large distributionof open pores, and the areas perforated by a thermal head vary, unevenamounts of ink are transferred, as described above. Therefore, thistechnique does not provide a solution to the problem that the dryingproperty of ink is low in portions where large amounts of ink aretransferred.

[0011] As a possible alternative solution to the above problem, anattempt has been made to decrease the viscosity of ink, to increase thepermeability of ink through the printing paper, thereby improving thedrying property. However, this is also not a practical solution, sincethe ink transfer amount increases with the decrease of ink viscosity,necessitating a further decrease in the perforation size of the stencilmaster, although there is a limit to how far the element density of athermal head can be increased.

[0012] As described above, to date no printing technique has beenproposed in stencil printing which can yield quick drying of printedmatter while maintaining the quality of the printed image, without theuse of a reactive ink such as an UV-curable ink.

SUMMARY OF THE INVENTION

[0013] Accordingly, an object of the present invention is to provide amicroporous stencil sheet that can control the ink transfer amount whena low-viscosity ink with high permeability through printing paper isused, thus improving the quick-drying property of ink in stencilprinting. Additionally, a further object of the present invention is toprovide a stencil master and a method for making a stencil master usingthe microporous stencil sheet, that can be used for stencil printingusing a low-viscosity ink.

[0014] The present inventors found that the use of an inelastic resinfilm with predetermined air permeability and thickness as a stencilsheet can control the ink transfer amount when a low-viscosity ink isused, and thereby accomplished the present invention.

[0015] Specifically, the present invention provides, as an aspect, amicroporous stencil sheet composed of an inelastic resin film for use instencil printing using a low-viscosity ink of a viscosity ranging from0.001to 1 Pa•s. This microporous stencil sheet has an air permeabilityranging from 1 to 600 seconds and a thickness ranging from 1 to 100 μm.The stencil sheet satisfying such requirements can appropriately controlthe ink transfer amount of a low-viscosity ink of a viscosity of 0.001to 1 Pa•s, which is highly permeable through printing paper.Consequently, the drying property of ink in printed matter can bemarkedly improved as compared with conventional inks (viscosity: 2 to 10Pa•s). Additionally, the ink transfer amount can be reduced, therebypreventing running of the ink on printed matter. The film constitutingthe stencil sheet is composed of an inelastic resin, and even when it ispressed against, for example, a thermal head for heat fusion, the filmis not deformed, thereby yielding an increased precision inmaster-making. Furthermore, no deformation of printed images occurs evenwhen stencil printing is performed at a high printing pressure.

[0016] In another aspect, the present invention provides a method formaking a stencil master using a microporous stencil sheet, in which themicroporous stencil sheet includes a thermoplastic resin film and has anair permeability ranging from 1 to 600 seconds and is of a thicknessranging from 1 to 100 μm. The method includes the step of closing themicropores of the microporous stencil sheet in a non-printing portion ofa desired printed image by heat fusion, thereby making itink-impermeable. By closing the micropores of the non-printing portionin this manner, a stencil master can be made as a mirror image so thatink permeate the micropores of the stencil sheet only in the printingportion.

[0017] In a further aspect, the present invention provides a method formaking a stencil master using a microporous stencil sheet, in which themicroporous stencil sheet has an air permeability ranging from 1 to 600seconds and is of a thickness ranging from 1 to 100 μm. This methodincludes the step of closing the micropores of the microporous stencilsheet in a non-printing portion of a desired printed image by depositionof a resin and/or wax, thereby making it ink-impermeable. In thismethod, the resin and/or wax is preferably deposited from a thermaltransfer sheet by fusion transfer, thereby closing the micropores.

[0018] The present invention also provides a stencil master obtained bythe methods mentioned above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic diagram showing master-making using athermal head, as an embodiment of a method for making a master accordingto the present invention.

[0020]FIG. 2 is a schematic diagram showing master-making usingsemiconductor laser, as an embodiment of a method for making a masteraccording to the present invention.

[0021]FIG. 3 is a schematic diagram showing master-making using athermal transfer sheet and thermal head, as an embodiment of a methodfor making a master according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The embodiments of the present invention will be illustrated indetail with reference to the attached drawings.

[0023] The microporous stencil sheet (hereinafter simply referred to as“stencil sheet”) of the present invention is composed of an inelasticresin film and is used for stencil printing using a low-viscosity ink ofa viscosity ranging from 0.001 to 1 Pa•s. This stencil sheet has an airpermeability ranging from 1 to 600 seconds and is of a thickness rangingfrom 1 to 100 μm.

[0024] Permeability of ink through printing paper increases with thedecrease of ink viscosity, and the ink cannot sufficiently permeateprinting paper if the viscosity exceeds 1 Pa•s. When a higher dryingproperty (quick drying property) of ink is required, the ink preferablyhas a viscosity of 0.1 Pa•s or less. The ink preferably has a surfacetension of 5×10⁻² N/m or less, or more preferably 4×10⁻² N/m or less,for yielding satisfactory ink-permeability. When pigments are used as acoloring agent of the ink, such pigments with larger particle size mayclog the micropores of stencil sheet, and pigments that have a smallparticle size and are dispersible are preferred. Alternatively, dyes canbe advantageously used for the coloring agent. Other components of theink such as vehicles and additives are not specifically limited. The inkis not limited to a stencil printing ink and also includes, for example,a water-based ink and an oil-based ink for use in the ink-jet process orin stamping.

[0025] To control the transfer amount of such low-viscosity ink, thestencil sheet has an air permeability within the range specified above.Herein, “air permeability” refers to the degree of air passing at apredetermined differential pressure and is determined using a Gurleydensometer according to the method described in Japanese IndustrialStandards (JIS) P 8117. If the air permeability exceeds 600 seconds, theink resists permeating the stencil sheet, which is impractical. If it isless than 1 second, it is difficult to control the ink transfer amount.

[0026] The stencil sheet is of a thickness within the above-specifiedrange. If the thickness of stencil sheet exceeds 100 μm,ink-permeability is reduced, causing insufficient uniformity in solidprint, and the stencil sheet becomes so firm as to deteriorate contactwith the heating means, such as a thermal head, and reduce operabilityin the master-making process. If the thickness is less than 1 μm, asufficient strength cannot be ensured and is not practical. In order toensure good operability, the stencil sheet preferably has a thickness of10 μm or more.

[0027] The micropores of the stencil sheet are pores that penetrate thestencil sheet from one face to the other and allow the ink to passtherethrough. The sizes of such micropores can be freely selecteddepending on the viscosity or surface tension of the low-viscosity inkused, but the mean pore size as determined using a mercury penetrationsystem porosimeter is preferably about 0.01 μm or more to ensuresatisfactory ink-permeability and is preferably about 10 μm or less toensure control of the transfer amount, when the ink is of a very lowviscosity. The mean pore size is more preferably in the range of about0.01 μm to about 1 μm. When the stencil sheet has micropores orderedfrom submicrons to microns in a high density, solid printing can beeasily performed even on printing paper that is resistant to the spreadof ink. Such micropores are preferably formed by stretching out(drawing) the inelastic resin film in at least one axial direction.

[0028] The porosity of the stencil sheet is not specifically limited,but is preferably 40% or more to ensure satisfactory ink-permeabilityand is preferably 90% or less to ensure satisfactory strength.

[0029] Regarding surface roughness, the stencil sheet preferably has aten point average roughness (height) Rz of 20 μm or less, as determinedaccording to the method described in JIS B 0601. Within this range, thestencil sheet has satisfactory contact with the heating means such as athermal head in master-making, with satisfactory transference from athermal transfer sheet in order to close the pores. Additionally, if thesurface roughness is excessively high, gaps (protrusions anddepressions) between the printing paper and the stencil sheet can becometoo large, with excess ink being supplied to such gaps therebyexcessively increasing the ink transfer amount.

[0030] The polymer material of the inelastic resin film constituting thestencil sheet is not specifically limited, but is preferably athermoplastic resin enabling master-making by heat fusion. Such resinsinclude, but are not limited to, polyethylenes, polypropylenes, andother polyolefins; poly(ethylene terephthalate), poly(butyleneterephthalate), and other polyesters; nylon-6,6 (polyamide-6,6),nylon-12 (polyamide 12) and other polyamides; poly(vinyl chloride),poly(vinylidene chloride) or copolymers thereof, and otherchlorine-containing resins; polytetrafluoroethylene,tetrafluoroethylene-ethylene copolymers, and other fluororesins. Ofthese, polyolefins are favored, of which polyethylenes are typicallypreferred. Each of these inelastic resins can be used either singly orin combination as a multi-layer structure.

[0031] Such preferred polyethylenes for use in the invented stencilsheet include, for example, polyethylenes for use in a filter membrane.Specifically, preferred polyethylenes are polyethylene homopolymershaving a density varying from high to low, as well as copolymers (linearcopolymerized polyethylenes) containing α-olefin unit such as propylene,butene, pentene, hexene, and octene. The content of comonomers in thesecopolymers is preferably about several mole percent (e.g., 4 molepercent or less) relative to ethylene unit. The inelastic resin film maycomprise component mixed with polyethylene, selectable from materialssuch as polypropylenes, high density polyethylene, medium densitypolyethylene, linear low density polyethylene, ethylene-propylenecopolymers, and other polyolefins. In this case, the content of theother polyolefins than polyethylene is preferably 30% by weight or less.

[0032] The molecular weight of the polymer is not specifically limitedand is freely selected depending on the type of resin from theviewpoints of, for example, fracture strength of the film andoperability in manufacture. For example, in the case of polyethylene,the polyethylene preferably has a weight average molecular weight (Mw),as determined by gel permeation chromatography using a standardpolystyrene calibration curve of 10×10⁴ or more to ensure satisfactoryfracture strength when the film is stretched out, and of 400×10⁴ or lessto ensure easy preparation of the resin solution in film formation. Theweight average molecular weight ranges more preferably from 20×10⁴ to70×10⁴, with typical preferences from 25×10⁴ to 50×10⁴. Alternatively,the weight average molecular weight can be maintained within thepreferred range by means of, for example, blending or multistagepolymerization.

[0033] The resin may further comprise, as required, various additives toa degree that does not hinder the formation of pores and thecharacteristics of the resulting stencil sheet. Such additives include,but are not limited to, dispersing agents, thixotropy-imparting agents,defoaming agents, leveling agents, diluents, plasticizers, antioxidants,fillers, and coloring agents.

[0034] A film (stencil sheet) using such a resin can be prepared orformed by a conventional method such as a casting process (T-dieprocess) using a molten polymer. Alternatively, the stencil sheet may beprepared by sintering resin particles onto the sheet.

[0035] The micropores in the resulting film can be formed by any methodsuch as a conventional microvoid formation method or solvent extractionmethod. For example, fine rips (microvoids) are formed in the boundariesbetween crystal regions and non-crystal regions by subjecting the filmto heat treatment to form microcrystals, and stretching out theresulting film in at least one axial direction. Such microvoids can alsobe formed by adding fillers to a molten polymer during the preparationof film, and stretching out the prepared film in at least one axialdirection to form microvoids in the filler portions. Alternatively, apolymer and a solvent are heated and melted to form a film, and the filmis cooled to phase-separate the solvent, and the resulting film isstretched out. In this procedure, the solvent is extracted prior to orsubsequent to stretching. Inorganic fillers may be added in thisprocedure to improve the dispersibility of the resin, thereby increasingpore-formation.

[0036] Films for use in the invented stencil sheet can be prepared inthe aforementioned manner, and are also commercially available as porousplastic films such as “HIPORE” available from Asahi Chemical IndustryCo., Ltd., “NF Sheet”(a microporous polypropylene sheet) and “PORUM”(amicroporous polyethylene sheet) both available from TokuyamaCorporation, “SUN-MAP”(a sintered polyethylene sheet), “MICROTEX”(atetrafluoroethylene resin sheet), and “BREATHRON”(a porous polyethylenesheet) all available from Nitto Denko Co., Ltd., “PERMILAN”(a porouspolyolefin sheet) available from Maruzen Polymer K. K., “ESPOIR”(aporous polyolefin sheet) available from Mitsui Chemicals, Inc., and“U-PORE”(a microporous polyethylene sheet) available from UbeIndustries, Ltd.

[0037] The stencil sheet is preferably a stretched film (drawn film).When a plastic film is stretched out in a predetermined direction duringmanufacture, the resulting film tends to shrink in the oppositedirection when heated after manufacture. Accordingly, when such astencil sheet that has been made heat-shrinkable by stretching is used,the micropores can be efficiently closed by heat fusion in master-makingusing the heat generated from a thermal head.

[0038] When heat fusion in master-making is performed usingelectromagnetic waves, the stencil sheet preferably has a photo-thermalconversion substance on the surface or inside of the sheet. Herein,“photo-thermal conversion substance” refers to a compound that cantransform light energy into heat energy upon irradiation ofelectromagnetic waves of an selectable wavelength, and includes anylight absorbers depending on the wavelength. The stencil sheet alsopreferably has an antistatic agent on the surface or inside of thesheet, in order to avoid the damaging transfer of the sheet caused bystatic electricity. In these embodiments, if the stencil sheet has aphoto-thermal conversion substance and/or antistatic agent on thesurface thereof, a layer containing at least such ingredient is formedon the surface of the stencil sheet by any means such as coating orpainting. If the stencil sheet has the ingredient inside thereof, theingredient is contained within the sheet, namely the resin constitutingthe film includes the ingredient in any form.

[0039] When the stencil master is made by irradiation withelectromagnetic waves, the preferred photo-thermal conversion substancesare substances that can efficiently transform light energy into heatenergy, such as carbon black, silicon carbide, silicon nitride, metallicpowders, metal oxides, inorganic pigments, organic pigments, and organicdyes. Of these, substances that can greatly absorb light of a specificregion of wavelength are typically preferred, such as phthalocyaninedyes, cyaninedyes, squaliriumdyes, andpolymethinedyes. Each of thesephoto-thermal conversion substances can be used singly or incombination.

[0040] Antistatic agents used for antistatic purpose include a varietyof surfactants. Such surfactants include, but are not limited to, fattyacid salts, higher alcohol sulfates, fatty acid amines, fatty acid amidesulfonates, fatty acid amide sulfates, aliphatic alcohol phosphates, andother anionic surfactants; aliphatic amines, quaternary ammonium salts,alkylpyridinium salts, and other cationic surfactants; polyoxyethylenealkyl ethers, polyoxyethylene alkyl phenol ethers, polyoxyethylene alkylesters, sorbitan alkyl esters, andothernonionicsurfactants;imidazolinederivatives, higher alkylamine (betaine) type amphotericsurfactants, sulfate phosphate type amphoteric surfactants, sulfonatetype amphoteric surfactants, and other amphoteric surfactants. Each ofthese substances can be used singly or in combination.

[0041] The photo-thermal conversion substance and antistatic agent maybe kneaded into inside of the resin before molding so that it isincluded within the film, or may be applied to the surface of the filmafter film-formation. The application technique is not specificallylimited, and, for example, these ingredients may be diluted with asolvent such as water or alcohol, and applied by spraying, dipping,brushing, or by the use of a roller coater, then the applied films areallowed to dry. The application operation can be performed in any stepbefore or after the formation of the micropores. The contents orapplication amounts of these ingredients are not specifically limited,and can be freely selected within a range in which the objects of theaddition of these ingredients can be sufficiently achieved withoutadversely affecting ink-permeability.

[0042] In a preferred embodiment, the stencil sheet has a releasinglayer on the surface thereof, which releasing layer contains a releaseagent selected from among silicone-based release agents, fluorine-basedrelease agents, wax-based release agents, and surfactant-based releaseagents. The formation of the releasing layer prevents fusion to aheating means and the heat shrinkage of a molten stencil sheet when thestencil master is made by closing the micropores of the stencil sheet byheat fusion method. Such release agents include, but are not limited to,silicone oil and other silicone-based release agents, fluorine-basedrelease agents, aqueous emulsion wax and other wax-based release agents,and phosphate-based surfactants and other surfactant-based releaseagents. Each of these release agents can be used singly or incombination.

[0043] Of these release agents, silicone-based release agents aredesirable, of which a release agent containing a silicone phosphateester as a principal component is typically preferred. The siliconephosphate ester concurrently has satisfactory lubricating property andreleasing property due to a silicon oil moiety, and satisfactoryantistatic property and adhesion to the substrate due to a phosphateester moiety. Additionally, the silicone phosphate ester is in the stateof liquid at ambient temperature, and no melting scum adheres to theheating means such as a thermal head. Specifically, the siliconephosphate ester can preferably be obtained by copolymerization betweendimethylpolysiloxane and a polyol such as a (polyoxyalkylene)olefinalcohol phosphate ester, and is represented by the following Formula(1):

[0044] wherein each of a and b is an independent integer of 1 or 2,where a plus b equals 3; Mis H, Na, K, Li or NH₄; R is represented bythe following Formula (2) when a polyol phosphate ester is introducedinto a side chain of dimethylpolysiloxane, and R is represented by thefollowing Formula (3) when a polyol phosphate ester is introduced intoan end of dimethylpolysiloxane:

[0045] wherein x, y and z each are independent values from 0 to 20, withx+y+z preferably values from 1 to 5; 1, m and n each are independentvalues from 0 to 200; R₁ is —(CH₂)_(p)CH₃ or a phenyl group, where p isa value from 0 to 10, and preferably p is 0 (i.e., R₁ is preferably amethyl group); R₂ is —(CH₂)₃—(OCH₂CH₂)_(s)—(OCH₂CH(CH₃))_(r)—(OCH₂CH₂)_(q)—OH, where q, r and s each are independentvalues from 0 to 20.

[0046] An example of the silicone phosphate ester is dimethiconecopolyol phosphate ester.

[0047] The release agent can be applied by any technique that is notspecifically limited. For example, an ingredient containing the releaseagent is dispersed or dissolved in a solvent freely selectable, and theresulting dispersion or solution being applied using a roller coater,gravure coater, reverse coater, or bar coater, and the solvent is thenvaporized. The application can be performed at any step before or afterthe formation of micropores. The coating amount of the releasing layerformed preferably ranges from about 0.001 to about 0.5 g/m², in order toyield a sufficient releasing property without hinderingink-permeability.

[0048] The releasing layer containing the release agent may furthercomprise additional ingredients within a range that does not adverselyaffect the objects of the present invention. Such additional ingredientsinclude, for example, antistatic agents as mentioned above, heat-fusiblesubstances, and binder resins.

[0049] Next, the embodiments of a method for making a stencil masteraccording to the invention will be described.

[0050] The master-making method is a method for making a stencil masterusing microporous stencil sheet, which comprises a thermoplastic resinfilm and has an air permeability ranging from 1 to 600 seconds and is ofa thickness ranging from 1 to 100 μm. This method includes the step ofclosing, by heat fusion, the micropores of the microporous stencil sheetin a non-printing portion of a desired printed image to form anink-impermeable portion. In the aforementioned master-making, themicropores in the non-printing portion should be closed at least in theface used for master-making and not be penetrating from one face of thestencil sheet to the other. That is, the non-master-making face of thestencil sheet may have pores left on its entire face.

[0051] As a heating means for use in the heat fusion operation, it ispreferable for a thermal head to be employed for ease and precisioncontrol in the master-making operation based on electronic data, therebyyielding clear printed images. The thermal head may be a line thermalhead or a serial thermal head.

[0052]FIG. 1 is a schematic diagram showing the master-making operationby heat fusion of a microporous stencil sheet using a thermal head.Microporous stencil sheet 10 is fed by a feeding roller that can befreely selected (not shown in the figure) through to an image formationunit composed of thermal head 20 and platen roller 21. The surface(master-making face) of microporous stencil sheet 10 is fused or meltedby heat from heating element 22 of thermal head 20 to form a closedportion (non-printing portion) 11 with closed micropores. Heatingelement 20 generates heat based on image signals. In this embodiment,microporous stencil sheet 10 has releasing layer 12 to avoid the fusionof the sheet to the thermal head.

[0053] In another embodiment, the heat fusion operation can be performedby irradiation with electromagnetic waves selectable from, for example,y-rays, X-rays, ultraviolet rays, visible rays, and laser light. In thiscase, it is required to use the stencil sheet having the photo-thermalconversion substance.

[0054]FIG. 2 is a schematic diagram showing the master-making operationby heat fusion of a microporous stencil sheet by irradiation withelectromagnetic waves. Infrared rays 31 which are condensed to a beamspot diameter of 50 μm is irradiated from semiconductor laser 30 ontothe surface of microporous stencil sheet 10, according to image signals.Then, heat energy is generated in a portion exposed to the infrared raysby the action of photo-thermal conversion substance layer 13 formed onthe surface of microporous stencil sheet 10, and the exposed portion ofstencil sheet 10 is fused by heat to form a closed portion (non-printingportion) 11.

[0055] In yet another embodiment of the master-making method, themicropores of a microporous stencil sheet are closed to form anink-impermeable portion by the deposition of a resin and/or a wax onto anon-printing portion of a desired printed image, instead of closing themicropores by heat fusion. In this procedure, the resin and/or wax ispreferably fused and transferred from a thermal transfer sheet using,for example, a thermal head to deposit the resin and/or wax onto anon-printing portion of the stencil sheet. By allowing part of thedeposited resin and/or wax to penetrate the micropores, the microporescan be closed more securely. Alternatively, the micropores can be closedby transferring a plain paper copier (PPC) toner and melting thetransferred toner.

[0056] The aforementioned resin includes, but is not limited to,polyolefin resins, vinyl acetate resins, polyacrylic resins, polyamideresins, polyester resins, polyurethane resins, polystyrene resins,petroleum resins, and rubber (gum) resins. The wax includes, forexample, natural waxes of vegetable origin, of animal origin, and ofmineral origin, petroleum waxes, synthesized hydrocarbon waxes, modifiedwaxes (denatured waxes), hydrogenated waxes, and fatty acid amide waxes.Each of these substances can be used singly or in combination.

[0057]FIG. 3 is a schematic diagram showing the master-making operationby the fusion and transference of resin from a thermal transfer sheetusing a thermal head, thereby making a master from a microporous stencilsheet. Microporous stencil sheet 10 is fed by a feeding roller which isfreely selectable (not shown in the figure) through to an image formingunit composed of thermal head 20 and platen roller 21. By means of theheat from heating element 22 of thermal head 20, which generates heatbased on image signals, part of thermal transfer layer 41 of thermaltransfer sheet 40 is transferred onto the surface (master-making face)of microporous stencil sheet 10 to form a thermal transferred region 42.A portion in which micropores are closed (non-printing portion) isformed under the thermal transferred region 42.

[0058] Stencil printing using the above-prepared master is performed inthe following manner. The master-making face of the stencil master issuperposed on printing paper, a low-viscosity ink is supplied from theopposite (non-master-making face) side of the stencil master under apressure, and the ink penetrates through the micropores of the stencilsheet which have not been closed (corresponding to the printing portion)and is transferred to the printing paper. Practical printing techniquesare not specifically limited. For example, continuous printing may beperformed by winding the master around the drum of a stencil printingdevice and supplying ink from inside the drum; or alternatively, pressprinting may be performed using a home-use portable stencil printingdevice such as “PRINT GOKKO”(trademark, a product of Riso KagakuCorporation). In this procedure, the ink can be supplied by impregnatingan article with the ink, which article is made of an open-celledmaterial and can be impregnated with the ink (e.g., sponge rubber madefrom a natural rubber or synthetic rubber, or formed synthetic resin),superposing the impregnated article with the non-master-making face ofthe master, and pressing the resulting laminate.

EXAMPLES

[0059] The present invention will be illustrated in further detail belowwith reference to several examples which are not intended to limit thescope of the invention.

[0060] As stencil sheets, microporous polyethylene films each having thethickness, mean pore size, porosity, air-permeability, and surfaceroughness indicated in Table 1 were used (Example 1: HIPORE H 3050,Example 2: HIPORE H 6022, Example 3: HIPORE 4050 U3, Example 4: U-POREupz 063, Example 5: HIPOREHN710, Example 6: U-PORE UP2015, Example 7:HIPORE X 9817, Example 8: HIPORE H 1100A, Comparative Example 1: HIPOREH 1080C, Comparative Example 2: SUN-MAP LC). “HIPORE”, “U-PORE” and“SUN-MAP” are trade names of Asahi Chemical Industry Co., Ltd., of UbeIndustries, Ltd., and of Nitto Denko Co., Ltd., respectively. Thepreparation method of each film is also shown in Table 1. The porositywas calculated from the volume and weight of a 10 cm square cut of thestencil sheet according to the following equation:

Porosity (%)={volume (cm³)−weight (g) /density}/volume (cm³)×100

[0061] 1. Evaluation on Covering Property

[0062] The micropores of each of the films in an area of about 15×15 cmwere closed (i.e., a blank master was made) in the following manner.

[0063] [Master-making 1]: The micropores of each film were closed with awax using a thermal transfer printer MD 5500 (manufactured by AlpsElectric Co., Ltd.) and a printer ribbon for MD 5500 (Model: MDC-FLK3).

[0064] [Master-making2]: A release agent solution comprising 1.0 part byweight of dimethicone copolyol phosphate ester (Pecosil PS-200, aproduct of Phoenix Chemical Incorp.) and 99.0 parts by weight ofisopropyl alcohol was applied onto each film using a wirebar to form areleasing layer with a dry weight of 0.05 g/m². The pores of each filmwith releasing layer were closed by heat fusion using PRINT GOKKOdigital master-making device (with a thermal head, a product of RisoKagaku Corporation).

[0065] Using the masters prepared by Master-making 1 and Master-making2, a printing test was performed in the following manner, and thecovering property (pore-closing property) of pores of each film wasvisually observed and evaluated according to the following criteria.

[0066] [Printing]: Each of the masters prepared by Master-making 1 andMaster-making 2 was set in a frame and was then set in PRINT GOKKO PG-11(trademark of Riso Kagaku Corporation). Stencil printing was performedusing this master and an ink-impregnated article. The ink-impregnatedarticle was obtained by impregnating an open-celled sponge (“Rubycell”,a product of Toyo Polymer Co., Ltd.) with a water-based dye ink(IC1-BKO5, ink for Epson IJ Printer) having a surface tension of3.2×10⁻² N/m and a viscosity of 3.2×10⁻³ Pa•s.

[0067] [Covering Property of Pores]

[0068] E: pores are entirely closed and the ink is not transferred toprinted matter

[0069] G: the master has some pinholes (some dots in printed matter) butto a degree negligible in practical use

[0070] NG: the master has noticeable pinholes (printed matter hasnoticeable dots)

[0071] 2. Printability Evaluation

[0072] Master-making and printing of each of the films were performed inthe following manner using a draft having both solid portion andcharacter portion from 6 to 10.5 points, and having an printed areapercentage of 25%.

[0073] [Master-making 1]: A wax image (a negative picture of draft) wasformed on each film using the aforementioned thermal transfer printerand printer ribbon to make a master.

[0074] [Master-making 2]: Master-making was performed by reversing anegative picture to a positive picture on each of the aforementionedfilms with releasing layer using the same PRINT GOKKO digitalmaster-making device as above.

[0075] Printing was performed in the same manner as above using theprepared master, and ink-permeability, image quality and ink-dryingproperty of the obtained printed matter were visually observed andevaluated according to the following criteria.

[0076] [Ink-permeability: evaluation of solid portion of printed matter]

[0077] E: ink permeates well, and solid areas are uniformly formed

[0078] G: solid areas are somewhat uneven but to a degree negligible inpractical use

[0079] NG: ink permeates insufficiently, and solid areas have noticeableunevenness in density

[0080] [Image Quality: evaluation of character portion of printedmatter]

[0081] E: printed image has no running of ink and is sharp

[0082] G: printed image has slight running and blurring but to a degreenegligible in practical use

[0083] NG: printed image has clearly observable running and blurring

[0084] [Ink Drying Property: the character portion of a printed matterwas rubbed with fingers, and smear was evaluated]

[0085] E: printed matter does not become dirty, with no smearing

[0086] G: printed matter becomes somewhat dirty but to a degreenegligible in practical use

[0087] NG: printed matter becomes noticeably dirty with smearing

[0088] The results are shown in Table 1. The results obtained inMaster-making method 1 and Master-making method 2 were the same in eachevaluation item. TABLE 1 Stencil Sheet Mean Result Air- Pore Surface InkThickness permeability Size Porosity roughness Manufacture Covering Ink-Image Drying (μm) (sec) (μm) (%) (Rz) Method Property permeabilityQuality Property Ex. 1 50 10 0.1 90 13.867 biaxial E E E E stretchingEx. 2 27 90 0.1 50 3.143 biaxial E E E E stretching Ex. 3 50 120 0.1 7013.467 biaxial E E E E stretching Ex. 4 24 250 0.19 49 1.197 uniaxial EE E E stretching Ex. 5 25 450 0.05 40 2.613 biaxial E G G E stretchingEx. 6 25 600 0.17 45 2.303 uniaxial E G G E stretching Ex. 7 39 600 0.0542 3.353 biaxial E G G E stretching Ex. 8 100 600 ≦0.l 55 2.327 biaxialE C G E stretching Com. 85 2000 ≦0.1 46 2.673 biaxial E NG NG E Ex.lstretching Com. 100 1 30 30 39.5 no NG E NG G Ex.2 stretching(sintering)

[0089] The stencil sheets according to the invented examples hadsatisfactory covering property and high ink-permeability, and theresulting printed matter exhibited satisfactory solid closed portion andcharacter portion. Additionally, even when the printed matter was rubbedwith fingers immediately after printing, the printed image was notdisturbed. In contrast, in Comparative Example 1 using a stencil sheetwith air-permeability of 2000 seconds, the ink-permeability was not goodand the image quality was deteriorated (blurred). In Comparative Example2 using a stencil sheet with air-permeability of less than 1 second, theink-permeability could not be sufficiently controlled, and an excessiveamount of ink permeated the master, causing running of ink in theprinted image. Additionally, the stencil sheet according to thiscomparative example had a poor covering property. This is probablybecause the film used was not a stretched film, and the coveringproperty was affected by, for example, the pore size and surfaceroughness of the film.

[0090] The present disclosure relates to subject matter contained inJapanese Patent Application No. 2000-188504, filed on Jun. 22, 2000, thedisclosure of which is expressly incorporated herein by reference in itsentirety.

[0091] Other embodiments and variations will be obvious to those skilledin the art, and this invention is not to be limited to the specificmatters stated above.

What is claimed is:
 1. A microporous stencil sheet composed of aninelastic resin film for use in stencil printing, wherein themicroporous stencil sheet has an air permeability ranging from 1 to 600seconds and a thickness ranging from 1 to 100 μm and is for use instencil printing using a low-viscosity ink with a viscosity ranging from0.001 to 1 Pa•s.
 2. A microporous stencil sheet according to claim 1,wherein the inelastic resin film is a thermoplastic resin film.
 3. Amicroporous stencil sheet according to claim 2, wherein the inelasticresin film is a polyolefin film.
 4. A microporous stencil sheetaccording to claim 1, wherein the micropores of the microporous stencilsheet have a mean pore size ranging from 0.01 to 10 μm.
 5. A microporousstencil sheet according to claim 1, wherein the microporous stencilsheet has micropores formed by stretching out the inelastic resin filmin at least one axial direction.
 6. A microporous stencil sheetaccording to claim 1, wherein the microporous stencil sheet has aphoto-thermal conversion substance on the surface or inside of themicroporous stencil sheet.
 7. A microporous stencil sheet according toclaim 1, wherein the microporous stencil sheet has an antistatic agenton the surface or inside of the microporous stencil sheet.
 8. Amicroporous stencil sheet according to claim 1, wherein the microporousstencil sheet has a releasing layer on its surface, the releasing layercomprising a release agent selected from the group consisting ofsilicone-based release agents, fluorine-based release agents, wax-basedrelease agents, and surfactant-based release agents.
 9. A microporousstencil sheet according to claim 8, wherein the releasing layercomprises a release agent containing a silicone phosphate ester as aprincipal component.
 10. A method for making a stencil master using amicroporous stencil sheet, the microporous stencil sheet comprising athermoplastic resin film and having an air permeability ranging from 1to 600 seconds and a thickness ranging from 1 to 100 μm, the methodcomprising the step of closing, by heat fusion, the micropores of themicroporous stencil sheet in a non-printing portion of a desired printedimage to form an ink-impermeable portion.
 11. A method according toclaim 10, wherein the heat fusion is performed by heat generated from athermal head.
 12. A method according to claim 10, wherein themicroporous stencil sheet has a photo-thermal conversion substance onthe surface or inside of the microporous stencil sheet, and wherein theheat fusion is performed by irradiation with electromagnetic waves. 13.A method for making a stencil master using a microporous stencil sheet,the microporous stencil sheet having an air permeability ranging from 1to 600 seconds and a thickness ranging from 1 to 100 μm, the methodcomprising the step of closing the micropores of the microporous stencilsheet in a non-printing portion of a desired printed image by depositionof a resin, wax or both to form an ink-impermeable portion.
 14. A methodaccording to claim 13, wherein the resin, wax or both are deposited froma thermal transfer sheet by fusion transfer.
 15. A stencil masterobtained by the method for making a stencil master according to claim10.
 16. A stencil master obtained by the method for making a stencilmaster according to claim 13.